FIG. 1 illustrates a reverse-conducting semiconductor device 200′ as disclosed in U.S. 2008/0135871 A1. The reverse-conducting semiconductor device 200′ as shown in FIG. 1 is a reverse-conducting insulated gate bipolar transistor (RC-IGBT), which includes, within one wafer 100, an insulated gate bipolar transistor with a built-in freewheeling diode. As shown in FIG. 1, such a reverse-conducting semiconductor device 200′ includes an n type base layer 101 with a first main side, which is the emitter side 104 of the integrated IGBT, and a second main side, which is the collector side 103 of the IGBT and which lies opposite the emitter side 104. A fourth p type layer 4 is arranged on the emitter side 104. Third n type layers 3 with a higher doping than the base layer 101 are arranged on the fourth layer 4.
A sixth electrically insulating layer 6 is arranged on the emitter side 104 and covers the fourth layer 4 and the base layer 101. The sixth layer 6 also partially covers the third layer 3. An electrically conductive fifth layer 5 is completely embedded in the sixth layer 6. No portion of the third or sixth layer 3, 6 is arranged above the central part of the fourth layer 4.
On this central part of the fourth layer 4, a first electrical contact 8 is arranged, which also covers the sixth layer 6. The first electrical contact 8 is in direct electrical contact with the third layer 3 and the fourth layer 4, but is electrically insulated from the fifth layer 5.
On the second main side, a seventh layer 7 formed as a buffer layer is arranged on the base layer 101. On the seventh layer 7, n type first layers 1 and p type second layers 2 are arranged alternately in a plane. The first layers 1 as well as the seventh layer 7 have a higher doping than the base layer 101.
A second electrical contact 9 is arranged on the collector side 103 and it covers the first and second layers 1, 2 and is in direct electrical contact with them.
In such a reverse-conducting semiconductor device 200′, a freewheeling diode is formed between the second electrical contact 9, part of which forms a cathode electrode in the diode, the n type first layer 1, which forms a cathode region in the diode, the base layer 101, part of which forms the diode base layer, the p type fourth layer 4, part of which forms an anode region in the diode, and the first electrical contact 8, which forms an anode in the diode.
An insulated gate bipolar transistor (IGBT) is formed between the second electrical contact 9, part of which forms the collector electrode in the IGBT, the p type second layer 2, which forms a collector region in the IGBT, the base layer 101, part of which forms the IGBT base layer, the fourth layer 4, part of which forms a p-base region in the IGBT, the third layer 3, which forms a n type source region in the IGBT, and the first electrical contact 8, which forms an emitter electrode. During the on-state of the IGBT, a channel is formed between the emitter electrode, the source region and the p-base region towards the n-base layer.
The n type first layer 1 includes a plurality of third regions 15 with a third region width 16. The p type second layer 2 includes a plurality of fourth region 25 with a fourth region width 26. The second layer 2 forms a continuous layer, in which each third region 15 is surrounded by the continuous second layer 2.
FIG. 2 illustrates the first and second layer 1, 2 over the whole wafer area through a cut along the line A-A from FIG. 1. This line is also indicated in FIG. 2 in order to show that the RC-IGBT 200′ does not have the same structure for the first and second layer 1, 2 over the whole plane of the wafer 100. In the upper part of the drawing (see line A-A), the structure of regularly arranged third regions 15 and fourth regions 25 is shown.
In the lower part of FIG. 2, it is shown that the second layer 2 further includes a fifth region 27 (surrounded by a dashed line in the drawing), which has a larger fifth region width 28, which is larger than the width 26 of any fourth region 25. The width 28 of a fifth region 27 plus the width 16 of a third region 15 is 1.5 to 5 times larger than the width 26 of a fourth region 25 plus the width 16 of a third region 15.
Such a structure is used in order to get large p doped areas for improvement of the on-state properties of the semiconductor device and by having areas, in which the p doped regions in form of fourth regions 25 are small compared to the fifth regions 27, the distance between the third regions 15 in the areas, in which third regions 15 are present, can be kept small. Thereby, the device can be used for higher currents.
However, due to the usage of third regions 15, each of which is surrounded by fourth regions 25, the possibilities to achieve good diode properties of the RC-IGBT are limited, because the area of the n type first layer 1, which is responsible for the diode properties, is small due to the geometrical conditions of such a device as known from US 2008/0135871 A1. If, for example, the width 16 of the third regions 15 is made as large as that of the fourth regions 25, the total n doped area is already not more than 25% of the whole area. By additionally introducing a larger p area as p doped fifth regions 27, the total n doped area is further reduced. If, on the other hand, the width 16 of the third regions 15 was enlarged as compared to the fourth regions 25, the IGBT properties would be worsened in an unacceptable manner, because snap back effects could occur.
US 2005/017290, EP 0683 30 and US 2008/093623 show known reverse conducting IGBTs with alternating n and p doped regions on the collector side of the devices. EP 0683530 discloses that the total area of the p doped regions is larger than the total area of the n doped regions.